The purpose of this project is to understand the functional characteristics of a protective monoclonal antibody against serotype A and C lipooligosaccharides from Moraxella catarrhalis.A monoclonal antibody, designed mAb 8E7 (IgG3), specific for M. catarrhalis lipooligosaccharide (LOS) was generated using BALB/c mice immunized with outer membrane vesicles of strain O35E. Enzyme-linked immunosorbent assay (ELISA) demonstrated that the mAb 8E7 could be prepared to a high titer against LOS of the homologous strain of M. catarrhalis and that it had bactericidal activity. MAb 8E7 reacted with M. catarrhalis serotype A and C LOSs, but not B LOS by ELISA and Western blotting. On the basis of published structures of LOSs, this suggests that the epitope recognized by mAb 8E7 is directed to a common sequence of either a-GlcNAc-(1>2)-b-Glc at the branch substituting position 4 of the trisubstituted Glc residue or a terminal tetrasaccharide at the branch substituting position 6 of the trisubstituted Glc residue. In a whole cell ELISA, mAb 8E7 reacted with 70% of 30 wild type strains tested. Immunoelectron microscopy demonstrated that mAb 8E7 reacted with a cell surface exposed epitope of LOS on strain O35E. The mAb 8E7 inhibited the adherence of strain O35E to Chang conjunctival epithelial cells by 90%. Passive immunization with mAb 8E7 could significantly enhance the clearance of strain O35E from mouse lungs in an mouse aerosol challenge model. This enhanced bacterial clearance was inhibited when mAb 8E7 was absorbed by serotype A LOS, indicating that the M. catarrhalis LOS-directed antibody may play a major role in enhancement of M. catarrhalis clearance from lungs. These data suggest mAb 8E7, which recognizes surface exposed LOS of M. catarrhalis, is a protective antibody against M. catarrhalis.